sync-preview: merge main into preview

[aidandaly24]

Description

Merge main into preview — includes harness Dockerfile and review script refactor.

⚠️ You MUST merge this PR using "Create a merge commit". NOT squash. NOT rebase.

Type of Change

• Other (please describe): sync main → preview

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	43.43%	8717 / 20067
🔵	Statements	42.78%	9228 / 21569
🔵	Functions	41.01%	1475 / 3596
🔵	Branches	40.66%	5683 / 13976

Generated in workflow #2198 for commit 7590650 by the Vitest Coverage Report Action

[agentcore-cli-automation]

parse_events in .github/harness/harness_review.py doesn't handle the validationException event type. The boto3 event stream for invoke_harness can emit messageStart, contentBlockStart, contentBlockDelta, contentBlockStop, messageStop, metadata, internalServerException, validationException, and runtimeClientError, but the current if/elif chain only yields five of those — validationException is silently dropped.

The previous SigV4-based parser treated any event with :message-type: exception as fatal and exited non-zero. With the boto3 rewrite we lost that fallthrough, so an in-stream validation error will now cause the workflow to exit 0 with no review posted and no error surfaced, which is a regression worth fixing before merge.

A couple of ways to fix:

1. Add an explicit elif "validationException" in event: branch in parse_events, and a matching handler in print_stream that closes the group, prints the error to stderr, and sys.exit(1) — mirroring the internalServerException / runtimeClientError handlers.

2. Generically handle any modeled exception event by yielding it as an error and exiting — e.g. check shape.metadata.get('exception') or just treat any top-level key that ends in Exception/Error as fatal. This would be more future-proof if the service adds new exception event types.

(Minor side note while you're in there: the new .github/harness/README.md describes harness_review.py as "Invokes the harness to review PRs (SigV4 + event stream)" — that parenthetical is stale now that the script uses the boto3 client.)